The prior art is replete with references to dyes which absorb in the infrared region of the visible spectrum. Such dyes find usefulness in such diverse fields as sensitizers for infrared sensitive phototgraphic emulsions, absorbers for optical storage systems, filters for infrared light, as laser dyes, etc.
Although there are many classes of dyes which absorb in the infrared, squarylium dyes, the class of dyes made by condensation of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) are of particular interest due to their properties, e.g. narrow absorptive band, high molar absorptivity, relative stability and minimal absorption of visible light.
However, as a class squarylium dyes absorb in the region of between about 500 to about 800 nanometers.
As the use of lasers has rapidly increased in recent years, so has the demand for laser-protective eyewear. The traditional methodology has been the use of color filter glass elements. However, these filters suffer from various severe shortcomings, including a broader bandwidth than is desired, limited transmission in the red spectral region, etc.
While squarylium dyes as a class do not suffer from these disadvantages, for protection against certain of the lasers in vogue today it is necessary that the dye possess peak absorption in a band in excess of 1000 nanometers, e.g. have a peak absorption of on the order of about 1055 nanometers in methylene chloride.
Stated simply, the task of this invention is to provide squarylium dyes which will possess a peak absorption in excess of 1000 nanometers in methylene chloride and which are further characterized by possessing a sharp peak, stability against degradation of protective absorbtivity upon contact with lasers, and minimal absorption of visible light whereby the dyes are characterized as being "clean".